1. Field of the Invention
The present invention relates to a lens system suitable for a video camera, digital camera, or camera using a film.
2. Related Background Art
In recent years, a variety of video cameras or digital cameras having solid-state image pickup elements have been developed.
Many video cameras or digital cameras are constructed by arranging various kinds of glass members such as low-pass filters and color filters between the rear end (final lens surface) of a photographing lens and an image pickup element.
For this reason, a photographing lens for a video camera or digital camera must have a long back focus as compared to its focal length.
A retrofocus type lens is conventionally known as a lens type with a long back focus. Japanese Patent Publication No. 46-24194 has proposed a retrofocus type lens formed from five lenses which include sequentially from the object side a first lens having negative refracting power and a meniscus shape with its convex surface facing the object side, a second lens having convex lens surfaces on both sides, a third lens having concave lens surfaces on both sides, a fourth lens having positive refracting power, and a fifth lens having convex lens surfaces on both sides. Numerical examples of this prior art have disclosed a lens system having a back focus about 1.3 times the focal length.
Additionally, along with the recent increase in number of pixels in solid-state image pickup elements, optical performance required of a photographing lens is becoming very high. A retrofocus type lens formed from five lenses, including negative, positive, negative, positive, and positive lenses from the object side, is proposed in, e.g., Japanese Laid-Open Patent Application No. 63-81414, 3-63613, 10-213742, 10-293246, or 2001-100091 as a lens system that attains both high imaging performance and a long back focus.
In such a lens system formed from five lenses, including negative, positive, negative, positive, and positive lenses from the object side, when the interval between the first and second lenses is reduced, it becomes difficult to obtain a sufficient back focus. If the back focus is inappropriately increased, the power (the reciprocal of the focal length) of the first lens increases to make it hard to correct aberrations. In addition, the performance degrades sensitively to the parallel and tilt decentering of the first lens. Conversely, when the interval between the first and second lenses is increased, the outer diameter of the first lens increases. This makes the back focus too long and the entire lens bulky. The retrofocus type lens proposed in Japanese Laid-Open Patent Application No. 63-81414 or 10-293246 can advantageously reduce the size of the lens system because the interval between the first and second lenses is relatively small, though aberration correction is difficult.
Furthermore, when the interval between the second and third lenses is reduced, it is difficult to insert an aperture stop between the second and third lenses.
Conversely, when the interval between the second and third lenses is increased, the diameter of the entire lens system increases, and the position through which an off-axis light beam will pass becomes relatively high. As a result, it becomes difficult to correct off-axis aberration.
The retrofocus type lens proposed in Japanese Laid-Open Patent Application No. 10-213742 or 2001-100091 maintains such an interval between the second and third lenses that an aperture stop can be inserted therebetween. However, that interval is not enough to insert a mechanical shutter means.
A photographing lens for a use in a video camera or digital camera must have a back focus long to some extent, as described above. However, a back focus more than necessity is not preferable because it increases the total length of the photographing lens.
In the retrofocus type lens proposed in Japanese Laid-Open Patent Application No. 3-63613, the back focus is as long as 1.2 or more times the focal length. Hence, the total length of the photographing lens tends to be large.
Another retrofocus type lens that is formed from five lenses, i.e., negative, positive, negative, positive, and positive lenses and tries to further improve its performance by using an aspherical surface is proposed in, e.g., Japanese Laid-Open Patent Application No. 9-166748.
In Japanese Laid-Open Patent Application No. 9-166748, aberrations are satisfactorily corrected by causing one surface of the fifth lens which is closest to the image side to have an aspherical surface. The refractive index of the material of the fifth lens is relatively as high as 1.6935.
Generally, as the refractive index of a glass material increases, the cost of the glass material itself becomes high, and the difficulty in forming an aspherical surface also increases.